The purpose of this work is the characterization of an obligate intracellular procaryote that has recently been isolated from monocytes of patients with acquired immune deficiency syndrome (AIDS). This agent is of interest because it has been shown to be very similar to Ehrlichia canis, the causative agent of canine ehrlichiosis, a disease in which the agent grows in monocytes of dogs resulting in an immuno-compromised condition that is most often fatal. E. canis is currently classified in the Rickettsiales, however ultrastructural studies have shown that the agent grows within a cytoplasmic endosome, a property shared with Chlamydia and Coxiella, not the Rickettsias. These observations suggest that the agent may be closely related to Chlamydia. The purpose of this study is to determine if the AIDS procaryote is related to the chlamydial group of organisms. The approach will be the following: (i) Propagate the agent from infected human monocytes into McCoy cells using standard methodology for the isolation and growth of Chlamydia. (ii) Perform ultrastructural studies of both infected cells and organisms isolated from infected cultures. These studies will describe the intracellular compartment of growth, agent morphology and its growth cycle properties. (iii) Immunological analysis with characterized monoclonal antibodies reactive against Chlamydia and Coxiella. (iv) DNA-DNA hybridizations with both chlamydial and Coxiella burnetii DNA and G+C ratios. (v) Metabolic and physiological studies to determine if the agent grows within the phagosome or phagolysosome of the cell to distinguish between Chlamydia and Coxiella. (vi) Introduction of the agent into various animal species with the goal of developing a model of immuno-compromised infections such as AIDS and ehrlichiosis.